Buying and Selling: Selling a Commodity for More Than the Specified Price
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2148
If a person gives a commodity to someone and specifies its price and says to him, ‘Sell this commodity for this price, and the more you sell the more your commission will be’, then whatever he gets above that price belongs to the owner of the commodity and the seller can only take his commission from the owner. However, if this is done in the form of a reward (juʿālah)[1] and the owner says, ‘If you sell this commodity for a price that is higher than that price, the extra amount belongs to you’, there is no problem.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling
1
The laws of juʿālah are stated in Chapter 15.